The present invention relates to a wet cylinder liner for a cylinder of an internal combustion engine, and particularly to the support for the liner in the cylinder head and to the cooling of the liner.
A cylinder liner of the "wet" type is a cylinder liner the outside of which is to at least a certain extent in direct contact with and flowed over by a cooling medium, usually the radiator fluid.
The invention concerns more specifically a further development of the type of wet cylinder liner usually called a "midstop" liner. This means a wet cylinder liner which is so placed in the cavity in the cylinder block in which the liner is inserted that the liner is supported in the radial direction of the cavity wall at a position located approximately halfway between the two ends of the liner, i.e. at the top at the upper end adjacent to the cylinder head, and at the bottom at the lower end, which faces into the crankcase. In the following text, the respective expressions "upper" and "lower" end or portion of the liner merely denote respectively the end/portion facing the cylinder head and the end/portion facing away from the cylinder head and downwards into the crankcase. They are thus completely independent of the actual orientation of the cylinder block in the engine compartment of the vehicle concerned or in relation to a vertical direction.
An example of a wet cylinder liner of the type initially referred to is the removable cylinder liner described in U.S. Pat. No. 4,244,330. This known liner is precisely of the "midstop" type. On its outer shell surface this known cylinder liner has a circumferential radial protrusion which bears on a circular supporting surface on the cavity wall in the cylinder block. From the heat transfer point of view, the radial protrusion divides the cylinder liner into an upper portion cooled by water and a lower portion not cooled by water. This design is advantageous from the heat distribution point of view because it enables effective cooling of the upper hottest portion of the liner while at the same time the lower portion of the liner is not cooled by water but maintains a heat created by combustion in the combustion chamber of the cylinder and thereby causes lower friction losses than would otherwise occur. In addition, cooling losses are less with a cylinder liner of the "midstop" type because the cooled surface of the liner is thereby smaller.
When the engine's cylinder head is fitted to the cylinder block (the engine block), the cylinder liner is deformed by the clamping forces which occur when the cylinder head is pressed against the cylinder head gasket and the upper portion of the cylinder liner by means of a number of bolts. The deformations do in principle occur over the whole liner but may only be harmful within the area where they are most severe, i.e. where the circular protrusion of the liner bears on the supporting surface in the cavity wall. In order to prevent these deformations resulting in damage to the piston moving to and from in the cylinder liner, it is necessary to provide the piston with relatively large clearances. However, this is disadvantageous because increased piston clearances result in corresponding increases in noise level, in fuel consumption and in propensity to carbon grinding. Piston ring wear also becomes greater and at the same time the risk of cylinder cavitation increases because of the piston's positive and negative pressures. These pressures are a consequence of the fact that the lower portion of the piston is subjected via the gudgeon pin to forces which are directed obliquely with respect to the axial center line of the cylinder liner and the piston. Owing to the piston clearance, these obliquely directed forces create the possibility of a certain tilting or oblique positioning of the piston in the liner.
The disadvantages outlined above of such a known cylinder liner of the "midstop" type are well-known to engine designers but no satisfactory solution to the underlying problems has hitherto appeared.